1. Field of the Invention
The present invention relates to a vehicle steering apparatus for providing a drive force of a steering actuator (such as an electric motor or a hydraulic cylinder) to a steering mechanism, such as a power steering apparatus or a steer-by-wire (SBW) system.
2. Description of the Related Art
Hitherto, a power steering apparatus constructed to transmit a drive force generated by an electric motor or a hydraulic cylinder to a steering mechanism for helping the steering operation is mounted to and used in a vehicle. For example, an electric power steering apparatus for transmitting a drive force of the electric motor via a gear mechanism to the steering mechanism or transmitting the same to the steering mechanism via a direct drive system is constructed in such a manner that a target value for driving the electric motor is set in accordance with a steering torque applied to a steering wheel, and the electric motor is controlled based on the set target value. The steering torque is detected by a torsion bar interposed between the steering wheel and the steering mechanism.
On the other hand, as a new vehicle steering apparatus, there is proposed a system in which a mechanical connection between the steering wheel and the steering mechanism for turning the steering tired-wheel is eliminated, and turning of the steering tired-wheel is achieved by detecting the operating angle of the steering wheel and, based on the detected results, providing a torque from the steering actuator to the steering mechanism (so called steer-by-wire (SBW) system. (See Patent Document 1 shown below).
The steering actuator is controlled by a control unit having a microcomputer. In other words, when the ignition switch of the vehicle is in an ON state, various detection signals of an operating angle sensor for detecting the operating angle of the steering wheel and a turning angle sensor for detecting the turning angle of the steering tired-wheel are input into the control unit. The control unit controls the steering actuator in such a manner that the turning angle of the steering tired-wheel corresponds to the operating angle of the steering wheel based on the input signals.
On the other hand, in order to provide an operation reaction force to a steering wheel, a reaction force actuator is provided on a steering shaft. The control unit provides the operation reaction force to the steering wheel by driving the reaction force actuator according to the operating angles and so on.
Reference documents relating to the present invention are shown below;
[Patent Document 1]JP-A-2000-198453[Patent Document 2]JP-A-10-324120[Patent Document 3]JP-A-11-20427
In the electronic power steering apparatus in the related art described above, the electric motor as a steering actuator is controlled according to the steering torque, and thus it can be said that the steering assistance according to the road conditions is performed as far as it is reflected to the steering torque. However, since the road conditions are reflected to the steering torque after being transmitted from the wheel through a mechanical link, detection may be delayed due to rattling or backrush of the steering mechanism, or torsion of a torsion bar, and thus delay of response in control cannot be avoided. That is, control of the steering actuator is not necessarily performed with a good responsibility.
In the same manner, in the steer-by-wire system as well, it is difficult to control the steering actuator while accurately reflecting the road conditions or other factors.
Accordingly, it is an object of the present invention to provide a vehicle steering apparatus, which is capable of detecting a load applied to the tire and performing steering control, in which the road conditions are accurately reflected, based on the detected load.
A fundamental factor for controlling the reaction force actuator (reaction force control) is to generate a self-aligning torque for reproducing a kickback from the road surface. Therefore, it is necessary to differentiate the setting for reaction force control for every individual vehicle, which causes elongation of the period of development.
More specifically, in the steer-by-wire system in which a steering gar and a handle column are not directly linked, the feedback of actual values of the axial force or a torque in the steering torque transmission system cannot be given to a driver via the steering wheel in contrast to the case of the normal-type steering mechanism in which the line from the steering wheel to the steering mechanism is mechanically linked. Therefore, the electrical feedback must be performed by converting the data on vehicle's behaviors into a self-aligning torque, or by controlling the reaction force actuator based on a torque of a pinion shaft that transmits a drive force of the steering actuator to a rack shaft. Therefore, in order to reproduce the self-aligning torque similar to the normal-type steering mechanism, vehicle adaptation technology for adjusting every individual vehicle is required.
In addition, in control of reaction force based on data of the vehicle's behavior or a torque of the pinion shaft, it is possible to transmit information on the road conditions (rutted road, cant road, wave road, snowy road) to the driver via the steering wheel. However, it is not possible to transmit transient information on the tire supporting surface (whether or not it is in the critical limit of grip). It is because information on the road condition is so called information in the low-frequency region, and thus it is sufficient when it is informed to the driver at a response speed in the order of 200 milliseconds, while transient information on the tire supporting surface have to be informed to the driver at a response speed in the order of several tens milliseconds for enabling the driver to take an appropriate reaction. However, in the reaction force control which is executed through a complex computing process based on data on the vehicle's behavior and so on, responsibility in the order of several tens milliseconds cannot be realized, and thus transient information on the tire supporting surface cannot be informed to the driver.
In addition, the driver uses consciously or unconsciously following information in order to drive a vehicle.    (1) Tire conditions (degree of wear: lowering of gripping force)    (2) Road conditions (slippery, irregular, wavy, rutted)    (3) Abnormality of vehicle, (abnormality, looseness, and wear of various parts in driving system or of underbody)    (4) Information received from traveling infrastructure
The driver senses such information visually, acoustically, or from vibrations of the seating surface and a reaction force generated when operating the operating terminals (such as steering wheel or control stick).
However, in the steer-by-wire system in which the steering gear and the handle column are not directly linked, in contrast to the normal-type steering mechanism in which the line from the steering wheel to the steering mechanism is mechanically linked, the feedback of the actual value of the axial force or the torque in the steering torque transmission system cannot be given to the driver via the steering wheel.
Therefore, transmission of information as described above to the driver tends to be insufficient. This is not a subject which is specific for the steer-by-wire system, but is a common subject in various technologies for improving the comfort or motility of vehicles (improving technology of a braking system, a driving system, and a suspension system in addition to the steer-by-wire technology), which may increase mental and physical stresses exerted to the driver during operation.
In view of such circumstances, it is an object of the present invention to provide the vehicle steering apparatus, which enables steering control in which the road conditions are accurately reflected based on the detected load applied to the tire. It is another object of the present invention to provide the vehicle steering apparatus in which application of the vehicle adaptation technology for every individual vehicle is reduced, so that the period of development can be shortened. It is a further object of the present invention is to provide the vehicle steering apparatus which realizes a reaction force control in which desirable transmission of transient information on tire supporting surface to the driver is achieved. It is one of the objects of the present invention is to provide the vehicle steering apparatus in which required information can be adequately informed to the driver and thus a comfortable driving environment is provided.